.177 or .22

There is an old debate about which airgun calibre is best: .177 or .22 hopefully this post will help you decide between the two

A .177 pellet travels at a greater velocity than the heavier .22 when fired from guns of identical power, but this does not mean that .177 is more powerful. For example:

.177 pellet weighing 8.4 grains, fired at 802 fps (244mps) = 12 foot pounds of kinetic energy.

.22 pellet weighing 16 grains, fired at 581 fps (177mps) = 12 foot pounds of kinetic energy.

The lighter .177 pellet has a flatter trajectory which means that hold over is less and more forgiving with range estimation than with the .22 over normal airgun ranges. The benefit of this is that targets are easier to hit with a .177 even if there are slight inaccuracies in range estimation; this is why .177 is the calibre of choice in Field Target and paper target disciplines.

.177 tradactory from 0-50 yards, the pellets in a 1 inch zone from 7.3 yards to 39.3 

.22 tradactory from 0-50 yards, the pellets in a inch zone from 5.5 to 30.4 yards

Pellets also have alot to do with it; the accuracy of the pellet means you can hit in the kill zone which is need for both calibres for a clean kill but more forgiving with the .22, the weight of the pellet also has a big effect as the lighter the pellet the flatter the trajectory for both but if the airgun is used for hunting you still need a heavy pellet so it has enough stopping power to kill.

Heavy pellets will also be affected by wind less. There is also a phrase: .177 for feather .22 for fur which you could also take into account

At the UK power level of 12ft/lbs the difference is fairly small, but as the power increases, the trajectory of both calibres flattens out so reducing the advantage that .177 would have over .22 at lower velocity. However, when a pellet approaches the speed of sound (which is about 1100 feet per second), it can become unstable, resulting in poor accuracy. This problem is almost exclusively confined to .177. The .22 really comes into its own as the velocity increases, when it’s heavier weight means that the pellets retain greatly superior down range-energy, and do not suffer from the loss of accuracy that can affect high velocity .177 pellets.

 summary of the information presented in a table